Crystal oscillators are commonly used in electronic systems, for example a mobile phone, to generate highly accurate clock signals. Referring to FIG. 1, a typical crystal oscillator 10 comprises an active part 11, which may be an amplifier, and a tank 16 coupled between an input 102 of the active part 11 and an output 104 of the active part 11. The input 102 of the active part 11 is coupled to a first terminal 12 of the crystal oscillator 10, and an output 104 of the active part 11 is coupled to a second terminal 14 of the crystal oscillator 10. The tank 16 comprises a crystal 13 coupled between the input and output 102, 104 of the active part 11, and a variable capacitor 15 also coupled between the input and output 102, 104 of the active part 11. The variable capacitor 15 may be omitted if tuning of the crystal oscillator 10 is not required, in which case the crystal 13 is the tank 16, as the crystal 13 has inherent capacitance and inductance. The tank 16 provides negative feedback from the output 104 of the active part 11 to the input 102 of the active part 11. Oscillation may be started by thermal noise in the crystal oscillator 10, and the noise is amplified by the active part 11. An oscillation signal is delivered at the second terminal 14 of the crystal oscillator 10. The amplitude of the oscillation signal is gradually built up to a final value, which may take several milliseconds. A resonant frequency of the tank 16 determines the frequency of oscillation.
To reduce power consumption, a crystal oscillator may be turned on only when needed. For example, in an idle mode of a mobile phone, the crystal oscillator may be turned on several times each second to listen to paging messages and then turned off again.
A drawback of crystal oscillators is the relatively long start-up time until the oscillation signal reaches its final amplitude, because crystals oscillators have a high Q-factor, where the Q-factor, also known as the quality factor, of the oscillator is defined asQ=2π(energy stored/energy dissipated per cycle of oscillation).Typically, a crystal oscillator has a Q-factor of the order 105. The higher the Q-factor, the longer the start-up time, and the higher the power consumption in the idle mode as the crystal oscillator needs to be turned for a longer time before it can be used.
There is a requirement for an improved oscillator circuit.